The invention relates to a system and a method for charging the energy storage cells of an energy storage device, particularly in a battery direct inverter circuit for supplying power to electric machines.
The trend is that, in the future, electronic systems that combine new energy storage technologies with electrical drive technology will be used increasingly both in stationary applications, such as wind power installations or solar installations, and in vehicles, such as hybrid or electric vehicles.
The supply of multiphase current to an electric machine is usually accomplished by a converter in the form of a pulse-controlled inverter. To this end, a DC voltage provided by a DC voltage intermediate circuit can be converted into a multiphase AC voltage, for example a three-phase AC voltage. In this case, the DC voltage intermediate circuit is powered by a line of battery modules connected up in series. In order to be able to meet the demands on power and energy for a respective application, a plurality of battery modules are frequently connected in series in a traction battery.
The document U.S. Pat. No. 5,642,275 A1 describes a battery system with an integrated inverter function. Systems of this kind are known by the name of multilevel cascaded inverter or else battery direct inverter (BDI). Such systems comprise DC sources in a plurality of energy storage module lines that can be connected directly to an electric machine or an electrical system. In this case, single-phase or polyphase supply voltages can be generated. The energy storage module lines have a plurality of series-connected energy storage modules in this case, each energy storage module having at least one battery cell and an associated controllable coupling unit that allows control signals to be taken as a basis for interrupting the respective energy storage module line or bypassing the respective associated at least one battery cell or connecting the respective associated at least one battery cell into the respective energy storage module line. Suitable actuation of the coupling units, e.g. using pulse width modulation, also allows suitable phase signals to be provided for controlling the phase output voltage, as a result of which it is possible to dispense with a separate pulse-controlled inverter. The pulse controlled inverter required for controlling the phase output voltage is therefore integrated in the BDI so to speak.
BDIs usually have a higher level of efficiency and a higher level of failsafety in comparison with conventional systems. Failsafety is ensured, inter alia, by the ability for faulty, failed or not fully effective battery cells to be disconnected from the energy supply lines by virtue of suitable bypass actuation of the coupling units. The phase output voltage of an energy storage module line can be varied, and, in particular, set in a stepped manner, by virtue of appropriate actuation of the coupling units. In this case, the step range of the output voltage is obtained from the voltage of a single energy storage module, with the maximum possible phase output voltage being determined by the sum of the voltages of all the energy storage modules in an energy storage module line.